feat: implement 'Erase Your Darlings' (Impermanence) root wipe

- Add @persist subvolume to BTRFS layout
- Implement automatic root-blank snapshotting during installation
- Add initrd rollback script to wipe root on every boot
- Configure persistence for core system state (NM, Bluetooth, SSH, NixOS config)

modules/system/impermanence.nix

@@ -47,8 +47,11 @@ in
         "/var/log"
         "/var/lib/nixos"
         "/var/lib/systemd/coredump"
+        "/var/lib/systemd/timesync"
         "/var/lib/bluetooth"
         "/etc/NetworkManager/system-connections"
+        "/etc/nixos"
+        "/etc/ssh"
       ];
       files = [
         "/etc/machine-id"